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A small reaction wheel viewed in profile A momentum/reaction wheel comprising part of a high-accuracy Conical Earth Sensor to maintain a satellite's precise attitude. A reaction wheel (RW) is an electric motor attached to a flywheel, which, when its rotation speed is changed, causes a counter-rotation proportionately through conservation of angular momentum. [1]
Momentum wheels are generally suspended on magnetic bearings to avoid bearing friction and breakdown problems. [5] Spacecraft Reaction wheels often use mechanical ball bearings. To maintain orientation in three dimensional space a minimum of three reaction wheels must be used, [6] with additional
The CMGs will eventually saturate (accumulating angular momentum to the point where they can accumulate no more), resulting in loss of effectiveness of the CMG array for control. Some kind of angular momentum management scheme (MMS) is necessary to allow the CMGs to hold a desired attitude and at the same time prevent CMG saturation.
Spacecraft rotational operations, such as turning to point in a new direction, are usually performed by angular momentum storage devices such as reaction wheels or control moment gyroscopes. It is generally preferable to use these devices instead of traditional thrusters , as they are powered by renewable electricity instead of by propellant ...
The speed is sometimes stabilised to prevent unwanted torque reaction. The internal friction losses are minimised by design. The momentum wheel(s) on a spacecraft is used in conjunction with reaction wheels. A set of momentum wheels 'translates' applied torque into a programmed direction. A momentum wheel can be configured as a CW or CCW unit.
The specific angular momentum of any conic orbit, h, is constant, and is equal to the product of radius and velocity at periapsis. At any other point in the orbit, it is equal to: [ 13 ] h = r v cos φ , {\displaystyle h=rv\cos \varphi ,} where φ is the flight path angle measured from the local horizontal (perpendicular to r .)
Reaction control systems are capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow control of rotation (roll, pitch, and yaw). [2] Reaction control systems often use combinations of large and small thrusters, to allow different levels of response.
There are also eight Monopropellant Rocket Engines (MRE-1), so called because they use only hydrazine as fuel. They are used for attitude control and momentum unloading of the reaction wheels. [2] [16] JWST has six reaction wheels for attitude control, spinning wheels that allow the orientation to be changed without using propellant to change ...